When inserting implants, such as, for example, artificial knee, elbow, finger or hip joints, it is necessary to position the implant on the bone as precisely as possible. To this end, precise incisions must be made to the bone structures that interface with the implant.
FIG. 17 schematically shows an incision block 1 being positioned on a femur bone K in accordance with the prior art. To this end, a guiding rod 100 is inserted into the bone K, wherein a positioning mechanism 101 for positioning and holding the incision block 1 in a desired position is provided at the outer end of the guiding rod 100. Once the incision block 1 has been moved to the desired position, the block 1 can be fixed to the bone using positioning pins 3, such as screws, nails and/or pins, for example, which can be inserted through holes 2 provided in the incision block 1 and into the bone K. After the incision block 1 has been successfully fixed to the bone K, a first incision plane S0, preferably as perpendicular as possible to the mechanical axis of the bone or slightly angled with respect to the mechanical axis of the bone, can be formed by means of an incision tool 4, as shown schematically in FIGS. 14A and 14B. In order to position the corresponding knee implant component on the bone K, additional incisions are made in additional incision planes S1 to S4 running obliquely to or angled with respect to the incision plane S0, as shown in FIG. 15. To this end, a second incision block 10 is placed onto the bone K at the first incision plane S0 and positioned by means of a suitable mechanism. The incision tool 4, which is guided by obliquely or laterally running slits of the second incision block 10, is used to make the additional lateral incisions to the bone.
A device 20 for determining the position of an incision block as shown in FIG. 16 is known from European Patent No. EP 1 190 676 A1 and corresponding U.S. Pat. No. 6,551,325 belonging to the Applicant, the disclosure of which is hereby incorporated by reference. The device includes a reference star 21 having a plurality of arms 21a, 21b, 21c extending therefrom. Spherical markers 22a, 22b, 22c, each having a reflective surface, are coupled to the arms, wherein two infrared cameras (not shown) detect infrared light reflected on the spherical markers 22a, 22b, 22c. Based on the incident and reflected infrared light, the cameras can ascertain the three-dimensional spatial position of the reference star 21.
The reference star 21 is fixedly connected to a base body 22, to which a rotatable element 23 is attached. Two plates 24a, 24b of different thicknesses are arranged on the outer end of the rotatable element 23 and can be inserted into a guiding slit, such as the guiding slit 1a of the tibial incision block 1 shown in FIGS. 12A to 12E. Alternatively, the plates 24a, 24b can be inserted or into one of the guiding slits II, III of the femoral incision block 1′ shown in FIG. 13.
In order to make the incision shown in FIG. 14, the incision block 1 is navigated to the desired point on the bone K and then is fixed to the bone K using fixing elements 3. An incision is made in the desired incision plane using the incision tool 4, as shown in FIG. 14B. A second incision block 10 then is placed on the incision plane S0 formed by the first incision, as shown schematically in FIG. 15, and incisions are made in the planes S1 through S4 using the guiding slits provided in the second incision block 10. Once the incisions are completed, an artificial joint can be placed on the bone and, provided the incision planes S0-S4 are correctly positioned, the artificial joint also will be correctly positioned.
Repeated use of the positioning device 20 described above, however, can lead to attrition in the area of the plates 24a and 24b shown in FIG. 16. Additionally, repeated use of the incision tool 4 may enlarge the guiding slits 1a, I,II,III. Plate attrition and/or enlargement of the slits can result in excessive clearance between the plates 24a, 24b and the walls of the slits 1a, I, II, III, thus allowing the reference star 21 to move with respect to the incision block. Movement of the reference star with respect to incision block can result in misalignment of the incision block 1, 1′ and, therefore, misalignment of the incision planes S0-S4.